System and method for monitoring particulate filter performance

ABSTRACT

In an apparatus having an internal combustion engine, an exhaust system for transporting engine exhaust from the engine is described. In one example, a particulate filter disposed in the exhaust system is assessed for degradation. The approach may be particularly useful for an image charge sensor.

TECHNICAL FIELD

The present application relates to the field of automotive emissioncontrol systems and methods.

BACKGROUND AND SUMMARY

Particulate matter filters are increasingly used in automotive emissionssystems for reducing particulate concentrations in engine exhaust.However, over time, such filters can suffer irreversible decreases intrapping efficiencies as the filter develops cracks due to uncontrolledtemperature excursion during the filter regeneration process, by meansof which the soot accumulated in the filter is burned off undercontrolled engine operating conditions. Losses in trapping efficiencymay result in increased particulate matter emissions well above theregulated limit.

Increasingly stringent particulate matter emissions standards andproposed government-mandated on-board diagnostic (OBD) requirements formonitoring the trapping efficiency of a particulate filter havestimulated much research into new techniques for monitoring particulatefilter performance. Currently, only laboratory grade instruments areavailable for particulate matter measurements. Such instrumentstypically measure particulate concentrations via optical, gravimetric orelectrical methods. These instruments typically require controlledoperating conditions and extensive calibration for proper functioning.Furthermore, some of these instruments, such as instruments that utilizeoptical measurement techniques, may require periodic cleaning.Therefore, these instruments may be too expensive and difficult to useunder normal automobile operating conditions to be a practical solutionto monitoring particulate emissions in automobiles. Furthermore, othermethods for detecting a particulate filter failure, such as thedifferential pressure method in which a pressure differential across thefilter is monitored, may not be suitable for detecting a failure of thefilter due to interference effects from ash loading in the filter.

The inventors herein have realized that the performance of a particulatefilter may be efficiently and effectively monitored by performing, in anapparatus having an internal combustion engine, an exhaust system fortransporting engine exhaust from the engine, a particulate filterdisposed in the exhaust system, and a particulate detector disposeddownstream of the particulate filter in the exhaust system, a method formonitoring a performance characteristic of the particulate filter,including detecting an increase in exhaust flow in the exhaust system;determining if the increase in the exhaust flow occurs at a rate equalto or greater than a threshold rate of change; and if the increase inthe exhaust flow is determined to have occurred at a rate equal to orgreater than the threshold rate of change, then determining from asignal received from the particulate detector if a value of a parameterrelated to particulate matter passing through the particulate filter hasa predetermined relationship to a predetermined threshold value thatindicates a reduction in filter trapping efficiency. In someembodiments, the particulate sensor may include an image charge sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic view of an exemplary embodiment of a dieselengine.

FIG. 2 shows a graphical representation of the output of an image chargesensor with digital output positioned upstream of a particulate filtercompared to an input mass air flow as a function of time for a pluralityof measurements.

FIG. 3 shows a graphical representation of the output of an image chargesensor positioned downstream of a particulate filter compared to anengine input mass air flow as a function of time for a plurality ofmeasurements.

FIG. 4 shows a graphical representation of the outputs of an imagecharge sensor positioned downstream of a properly functioningparticulate filter and an image charge sensor positioned downstream ofan improperly functioning particulate filter compared to an engine inputmass air flow as a function of time.

FIG. 5 shows a flow diagram of an exemplary embodiment of a method ofmonitoring a performance of a particulate filter.

FIG. 6 shows a graphical representation of the outputs of an upstreamparticulate sensor and a downstream particulate sensor compared to anengine exhaust flow and derivative of the engine exhaust flow as afunction of time.

FIG. 7 shows a schematic view of another exemplary embodiment of adiesel engine.

FIG. 8 shows a flow diagram of another exemplary embodiment of a methodof monitoring a performance of a particulate filter.

FIG. 9 shows a graphical representation of the output of a downstreamparticulate sensor compared to an engine exhaust flow and derivativethereof as a function of time.

DETAILED DESCRIPTION OF THE DEPICTED EMBODIMENTS

FIG. 1 shows an exemplary embodiment of a diesel engine system generallyat 10. Specifically, internal combustion engine 10 comprises a pluralityof cylinders, one cylinder of which is shown in FIG. 1. Engine 10 iscontrolled by electronic engine controller 12. Engine 10 includescombustion chamber 14 and cylinder walls 16 with piston 18 positionedtherein and connected to crankshaft 20. Combustion chamber 14communicates with an intake manifold 22 and an exhaust manifold 24 viarespective intake valve 26 and exhaust valve 28.

Intake manifold 22 communicates with throttle body 30 via throttle plate32. In one embodiment, an electronically controlled throttle can beused. In one embodiment, the throttle is electronically controlled toperiodically, or continuously, maintain a specified vacuum level inintake manifold 22. Alternatively, throttle body 30 and throttle plate32 may be omitted.

Combustion chamber 14 is also shown having fuel injector 34 coupledthereto for delivering fuel in proportion to the pulse width of signal(fpw) from controller 12. Fuel is delivered to fuel injector 34 by aconventional fuel system (not shown) including a fuel tank, fuel pump,and fuel rail (not shown). In the case of direct injection engines, asshown in FIG. 1, a high pressure fuel system is used such as a commonrail system. However, there are several other fuel systems that could beused as well, including but not limited to EUI, HEUI, etc.

In the depicted embodiment, controller 12 is a conventionalmicrocomputer, and includes a microprocessor unit 40, input/output ports42, electronic memory 44, which may be an electronically programmablememory in this particular example, random access memory 46, and aconventional data bus.

Controller 12 receives various signals from sensors coupled to engine10, including but not limited to: measurements of inducted mass airflow(MAF) from mass airflow sensor 50 coupled to the air filter [A on FIG.1]; engine coolant temperature (ECT) from temperature sensor 52 coupledto cooling jacket 54; a measurement of manifold pressure (MAP) frommanifold pressure sensor 56 coupled to intake manifold 22; a measurementof throttle position (TP) from throttle position sensor 58 coupled tothrottle plate 32; and a profile ignition pickup signal (PIP) from Halleffect (or variable reluctance) sensor 60 coupled to crankshaft 20indicating engine speed.

Engine 10 may include an exhaust gas recirculation (EGR) system to helplower NO_(x) and other emissions. For example, engine 10 may include ahigh pressure EGR system in which exhaust gas is delivered to intakemanifold 22 by a high pressure EGR tube 70 communicating with exhaustmanifold 24 at a location upstream of an exhaust turbine 90 a of acompression device 90, and communicating with intake manifold 22 at alocation downstream of an intake compressor 90 b of compression device90. The depicted high pressure EGR system includes high pressure EGRvalve assembly 72 located in high pressure EGR tube 70. Exhaust gastravels from exhaust manifold 24 first through high pressure EGR valveassembly 72, and then to intake manifold 22. An EGR cooler [shown at Yin FIG. 1] may be located in high pressure EGR tube 70 to coolrecirculated exhaust gases before entering the intake manifold. Coolingis typically done using engine water, but an air-to-air heat exchangermay also be used.

Engine 10 may also include a low pressure EGR system. The depicted lowpressure EGR system includes a low pressure EGR tube 170 communicatingwith exhaust manifold 22 at a location downstream of exhaust turbine 90a, and communicating with intake manifold 22 at a location upstream ofintake compressor 90 b. A low pressure valve assembly 172 is located inlow pressure EGR tube 170. Exhaust gas in the low pressure EGR looptravels from turbine 90 a through a catalytic device 82 (for example, adiesel oxidation catalyst and/or NO_(x) trap) and a diesel particulatefilter 80 before entering low pressure EGR tube 170. A low pressure EGRcooler Ya may be positioned along low pressure EGR tube 170.

High pressure EGR valve assembly 72 and low pressure EGR valve assembly172 each has a valve (not shown) for controlling a variable arearestriction in high pressure EGR tube 70 and low pressure EGR tube 170,which thereby controls flow of high and low pressure EGR, respectively.

Vacuum regulators 74 and 174 are coupled to high pressure EGR valveassembly 72 and low pressure EGR valve assembly 172, respectively.Vacuum regulators 74 and 174 receive actuation signals from controller12 for controlling the valve positions of high pressure EGR valveassembly 72 and low pressure EGR valve assembly 172. In a preferredembodiment, high pressure EGR valve assembly 72 and low pressure EGRvalve assembly 172 are vacuum actuated valves. However, any type of flowcontrol valve or valves may be used such as, for example, an electricalsolenoid powered valve or a stepper motor powered valve.

A particulate sensor 176 may be placed in the exhaust system betweenparticulate filter 80 and the tailpipe to monitor particulate emissions.Likewise, a second particulate sensor 178 may be placed upstream ofparticulate filter 80. Particulate sensor 178 may be placed eitherbetween catalytic device 82 and particulate filter 80 (as depicted), ormay be placed upstream of catalytic device 82. Particulate sensor 176may be referred to herein as “downstream particulate sensor 176” andparticulate sensor 178 may be referred to herein as “upstreamparticulate sensor 178.”

Compression device 90 may be a turbocharger or any other such device.The depicted compression device 90 has a turbine 90 a coupled in theexhaust manifold 24 and a compressor 90 b coupled in the intake manifold22 via an intercooler [shown at X in FIG. 1], which is typically anair-to-air heat exchanger, but could be water cooled. Turbine 90 a istypically coupled to compressor 90 b via a drive shaft 92. (This couldalso be a sequential turbocharger arrangement, single VGT, twin VGTs, orany other arrangement of turbochargers that could be used).

Further, drive pedal 94 is shown along with a driver's foot 95. Pedalposition sensor (pps) 96 measures angular position of the driveractuated pedal.

Further, engine 10 may also include exhaust air/fuel ratio sensors (notshown). For example, either a 2-state EGO sensor or a linear UEGO sensorcan be used. Either of these can be placed in the exhaust manifold 24,or downstream of devices 80, 82 or 90. It will be understood that thedepicted diesel engine 10 is shown only for the purpose of example andthat the systems and methods described herein may be implemented in orapplied to any other suitable engine having any suitable componentsand/or arrangement of components.

As described above, particulate sensors that are currently used fordetecting particulate concentrations in automotive exhaust are generallylaboratory grade instruments that may require controlled operatingconditions and extensive calibration. As such, these sensors may not besuitable for on-board use in commercially produced automobiles. As analternative to these laboratory grade sensors, downstream sensor 176 andupstream sensor 178 may be image charge sensors. Image charge sensorsare typically used for measuring the presence and/or concentration ofsolids such as solid chemicals, food products, dust, etc. in a fluidflow, can be manufactured at low cost, and can better survive exposed tothe harsh diesel exhaust than currently available laboratory gradeexhaust particulate detectors. Furthermore, image charge sensors mayrequire less extensive calibration than laboratory grade sensors. Anysuitable image charge sensor may be used as downstream sensor 176 and/orupstream sensor 178. Examples of suitable charge image sensors include,but are not limited to, the PCME DA550 PM particulate matter sensor,available from PCME of Cambridgeshire, UK.

Image charge sensors generally include an electrically conductive body,such as a rod, or a plate, or a probe of other shape, which is insulatedfrom the surrounding environment. The probe is placed in a flow of fluidand develops a time-dependent induced charge caused by a time-dependentflow of charged particles moving in front of the probe in the fluidflow. Such time-dependent charge can be detected by means of a chargeamplifier, or similar electronic devices, connected to the probe. It isknown that particulate matter flowing directly from the engine tends tohave an approximately equal distribution of positively-charged andnegatively-charged particles created during the combustion and blow-byprocess. Therefore, such particulate matter may be difficult to detectwith an image charge sensor because the device would need to have a highfrequency response. However, a portion of the particulate matterproduced by the engine is, over time, deposited on the exhaust systemwall, with the largest portion of these deposits upstream of particulatefilter 80. During a sudden exhaust flow increase, soot particles aredislodged from the exhaust system walls, thereby temporarily increasingthe particulate matter concentration in the exhaust gas. It has beenfound that the particles detached from the walls are tribologicallycharged with primarily one polarity of charge, as opposed to theapproximately even distribution of charges found in particulate matterflowing directly from the engine. This transient rush of chargedparticulate matter dislodged from the walls and moving in front of theconductive probe of an image charge sensor inserted in the exhaust gasflow has been found to generate a signal detectable at relatively lowfrequency.

FIG. 2 shows, generally at 200, a plot of the signals from an imagecharge sensor positioned upstream of a particulate filter and downstreamof a plate configured to simulate the blocking effect of a dieseloxidation catalyst on an exhaust flow. The upper set of lines, indicatedat 202, show the flow through a mass air flow sensor positioned at theengine intake at an engine-speed/torque of 1000 rpm/200 N*m. Thestep-like device response is an artifact of the digital signalconditioning of the specific image charge sensor used in the example andis not relevant to the methods taught herein. Each set of flow datashows a sudden increase in mass air flow, indicated at 204, caused byshutting off a flow of EGR to the engine. This also causes an increasein exhaust flow. For example, turning the EGR off reduces theparticulate number approximately by 20 times while the mass air flowroughly doubles, with most of the flow change occurring within 0.2 s.

As can be seen in FIG. 2, the increase in mass air flow and exhaust flowis accompanied by a peak in the signal from the image charge sensor,such as that indicated at 206. Any other engine operating parameterindicative of an increase in exhaust flow, including but not limited topedal position, engine speed, indicated torque, and/or a rate of changeof any other suitable engine parameter, can be used as alternative tothe mass air flow signal relied upon in this embodiment. The peak in thesignal from the image charge sensor is generally asymmetrical, and has awidth on the order of 2-3 s at half height. FIG. 2 also shows thatsimilar increases in exhaust flow can be accompanied by signals ofdifferent magnitudes from the image charge sensor, as illustrated by thedifferent magnitudes of peaks 206, 208 and 210. Without wishing to bebound by theory, this may be due to different amounts of particulatematter accumulating on the exhaust system wall between different exhaustburst events.

The quantity of particles dislodged from the exhaust system wall perunit time may depend on the rate at which the exhaust flow increases,and/or on the length of time prior to such exhaust transient duringwhich the exhaust flow was relatively constant or decreasing and duringwhich the exhaust flow contained a relatively high soot concentration,which favors soot accumulation on the wall.

FIG. 2 further shows that, under steady state condition (constantexhaust mass flow) the device signal output appears too small toevaluate the correlation between sensor output and particulate matterconcentration due to the approximately equal distribution of particlesof opposite polarities in the particulate matter flowing directly fromengine 10.

The signal from the image charge sensor may rise and decay relativelyslowly during a sudden exhaust flow increase event. Therefore, thesignal from the image charge sensor may be sampled at a lower frequencycompared to a sensor that attempts to detect particles dislodged fromthe exhaust system wall by a combustion/blow down event (i.e. thecombustion and exhaust strokes of the engine). For example, the sensorused to acquire the data shown in FIG. 2 was sampled at a frequency ofapproximately 10 Hz while the digital signal conditioning of the devicewas updating at less frequent rate (roughly 1 Hz). This is evidenced bythe stair-step shaped decay curve for each sensor signal plot. While thedepicted data shows a digital output device with a response time ofapproximately 1 s, it will be appreciated that any other suitablesampling frequency response time, either greater than or less than 1 s,also may be used.

FIG. 3 shows, generally at 300, a plot of the signals from an imagecharge sensor positioned downstream of a particulate filter during asudden exhaust flow increase. It can be seen that the image chargesensor signals associated with the exhaust transient, while attenuatedrelative to the signals from the upstream image charge sensor shown inFIG. 2, still show significantly stronger peak signals 302 than thelevel of noise during steady state engine operation. This indicates thatthe image charge sensor is effective in detecting particulate matterdownstream of the particulate filter, and that the sensor is effectivein distinguishing different concentrations of particulate matter in theexhaust gas. Particulate matter that passes through the particulatefilter may remain charged because the particles do not hit the filterwalls, and thus may produce a signal on a sensor positioned downstreamof the particulate filter.

FIG. 4 shows, generally at 400, a comparison of a signal from an imagecharge sensor positioned upstream of a particulate filter 402, a signalfrom an image charge sensor positioned downstream of a properlyfunctioning particulate filter 404, and a signal from an image chargesensor positioned downstream of a particulate filter 406 that has beenartificially deteriorated by creating several flow-through channels(>10% of the wall-flow channels) that have very low trapping efficiencycompared to a mass air flow rate as a function of time. The peaks in themass air flow rate shown in FIG. 4 were produced by laboratorycontrolled speed/torque transients, which produce much larger changes inexhaust flow than those caused by turning EGR flows on or off. Thesensors were sampled at 1 Hz to obtain the data in the sensor responseplots.

FIG. 4 shows that the response from the sensor downstream of thepartially drilled filter 406 shows a similar, though somewhatattenuated, response as that of the sensor located upstream of thefilter 402. On the other hand, the response from the sensor downstreamof the properly functioning filter 404 shows an even greaterattenuation, which indicates that the tested image charge sensor iscapable of detecting a difference between a properly functioning and animproperly functioning particulate filter. FIG. 4 also shows that theresponse from each image charge sensor is larger during the firstspeed/torque transient relative to the responses from the otherspeed/torque transients, possibly due to a larger amount of particulatematter being deposited on the exhaust system wall before the firsttransient.

FIG. 5 shows, generally at 500, a flow diagram of an exemplaryembodiment of a method of monitoring a performance of an engine exhaustparticulate filter. Generally, method 500 infers that the particulatefilter trapping efficiency has deteriorated below a certain thresholdbased on the signals of downstream particulate sensor 176 and upstreamparticulate sensor 178, as well as on the rate of change of the exhaustflow, which may be directly measured or derived from the engine intakemass air flow and fuel mass injected. The determination is based on acount of charged exhaust particle bursts that produce an image chargesignal above a calibratable threshold value in downstream particulatesensor 176 compared to a count of charged particle bursts producing animage charge signal above a calibratable threshold value in upstreamparticle detector 178, such signals occurring in conjunction with arapid exhaust flow increase.

More specifically, method 500 first includes, at 502, initializing thecounting variables and constants used in the method. Next, at 504,method 500 includes sampling the various engine sensors and inputtingthe sensor readings into controller 12. After inputting the sensorreadings, method 500 next includes determining at 506 whether theexhaust flow is increasing at a rate greater than a predeterminedthreshold (“Th_df”). If not, method 500 loops back to 504 to againsample and input sensor readings. The comparison of the rate of exhaustflow increase to the threshold value helps to ensure that only thoseincreases in exhaust flow with a high enough rate of change to dislodgesignificant amounts of particulate matter from the exhaust system wallsare used in the particulate sensor diagnostic.

On the other hand, if it is determined at 506 that the exhaust flow isincreasing at a rate greater than Th_df, then method 500 includes, at508, determining whether the signal received from the upstream sensor(“upSen”) is greater than a predetermined upstream sensor thresholdvalue Th_up. If the signal upSen is greater than the threshold Th_up,then method 500 includes increasing, at 510, a time counting variableTmup by a selected value, which is one in the depicted embodiment, andthen proceeding to step 512. On the other hand, if the signal upSen isnot greater than the threshold Th_up, then method 500 proceeds to step512 without increasing the timing variable Tmup. The use of the countingvariable Tmup allows method 500 to require that the signal from theupstream sensor stay above the threshold sensor value Th_up for apredetermined period of time before using the particulate sensor readingfor diagnostic purposes.

At 512, Tmup is compared to a threshold time limit Tm_up. If Tmup isgreater than Tm_up (i.e. if sufficient time has passed for the increasein particulate matter to be used for the diagnostic), then a countervariable up_Counter, which used to count a number of particulate matterpeaks that have passed the sensor output level threshold Th_up and thetime threshold Tm_up, is increased by one. Additionally, the timecounting variable Tmup is reset to zero. On the other hand, if Tmup isnot greater than the threshold Tm_up, then method 500 eventually cyclesback to 502 without increasing up_Counter, and without resetting thetime counting variable Tmup.

Any suitable values may be used for the threshold Tm_up and for theinitial value of the counter variable Tmup. For example, where thesensor has a sampling rate of approximately 100 ms, Tm_up may have avalue of 1 s, and Tmup may have an initial value of 0 s. With thesevalues, an exhaust burst would cause an increase in up_Counter only ifthe change in exhaust flow and the upstream sensor output stay above thethreshold values for one second or more. Furthermore, up_Counter wouldbe increased one additional increment for each additional second thesevalues remain above the threshold values.

Next, similar steps to those performed regarding the upstream sensor areperformed regarding the downstream sensor. First, at 514, a signal fromthe downstream sensor dnSen is compared to a threshold downstream sensorvalue Th_dn. If the sensor signal dnSen is greater than the thresholdTh_dn, then a time counting variable Tmdn is increased by a value of oneat 516, and method 500 continues to 518. On the other hand, if thesensor signal dnSen is not greater than the threshold Th_dn, then thecounter is not increased. In either case, method 500 next includescomparing, at 518, the time counting variable Tmdn to a predeterminedtime threshold Tm_dn to see whether the signal dnSen from the downstreamsensor has exceeded the threshold Th_dn for a sufficient time to becounted for diagnostic purposes. If so, then a counter variabledn_Counter, used to count a number of particulate matter peaks that havepassed the sensor output level threshold Th_dn, and the time thresholdTm_up, is increased by one. On the other hand, if Tmup is not greaterthan the threshold Tm_up, then method 500 cycles back to 502 withoutincreasing dn_Counter.

Method 500 continues to loop, as indicated at 520, until up_Counterexceeds a predetermined maximum number Nmax, which may be based upon anumber of exhaust burst events that is considered to provide sufficientdata for the diagnostic. Once up_Counter exceeds Nmax, a ratio (whichmay be referred to as the “leakage index”) of dn_Counter/up_Counter isdetermined and compared, at 522, to a leakage index threshold thatrepresents a pass/fail threshold for particulate matter performance. Ifthe leakage index is greater than the predetermined leakage indexthreshold, then, at 524, the particulate filter is determined to havefailed. The failure indication signal is then used to activate theMalfunction Indicator Light (MIL). On the other hand, if the leakageindex is less than the predetermined leakage index threshold, then theparticulate filter is determined to have passed the diagnostic. Method500 may be performed at any suitable time and may be repeated at anysuitable interval.

Method 500 offers the advantage that the diagnostic is not dependentupon precise sensor measurements, but instead merely tests that thesignals from the sensors meet various thresholds. Furthermore, the useof up_Counter and dn_Counter to count the number of events that pass theupstream and downstream sensor magnitude and duration thresholds makesthe diagnostic less dependent upon the actual sensor readings, which maypermit the use of less expensive and simpler sensors such as imagecharge sensors. Additionally, performing the diagnostic only aftersudden increases in exhaust flow allows other spurious signals inducedon the sensor to be readily rejected, thereby increasing the detectionlimit of particles leaked through the particulate filter.

FIG. 6 shows, generally at 600, a schematic representation of a signalfrom the upstream particulate sensor 602, a signal from the downstreamparticulate sensor 604, a rate of change in exhaust flow 606, and aderivative of the rate of change of exhaust flow 608 as a function oftime. Furthermore, an exemplary value of the flow threshold Th_df isshown at 610, an exemplary value of the upstream sensor threshold Th_upis shown at 612, and an exemplary value of the downstream sensorthreshold is shown at 614. Additionally, four exemplary exhaust burstevents are shown at 620, 622, 624 and 626. Depending upon the valuesused for the time thresholds Tm_up and Tm_dn, exhaust flow bursts 620,622 and 626 may be sufficient to produce an increase in up_Counterand/or dn_Counter. On the other hand, exhaust flow burst 624 has tooslow a rate of change to exceed the threshold Th_df, and therefore wouldnot cause an increase in up_Counter and dn_Counter even if the sensorsignals are greater than the threshold sensor signal values during thebursts.

Changes in the threshold values may have a relatively large effect onthe magnitude of the leakage index determined from up_Counter anddn_Counter. Table I shows a comparison of the number of experimentallycontrolled exhaust flow increases that triggered particulate matterbursts sufficient to exceed downstream sensor thresholds Th_dn of 0.4 V,0.42 V and 0.44 V.

TABLE I Number of peaks Time above threshold above threshold ThresholdPre- Post 50% Post good Pre- Post 50% Post good (V) filter leaky filterfilter filter leaky filter filter 0.4 169 157 150 150 0.42 31 8 2 10 4 10.44 20 0 0 9 1 0

As can be seen in Table I, either too high or too low a threshold forthe downstream particulate sensor threshold Th_dn may make increase thedifficulty of detecting a degraded or failed particulate filter.Therefore, a suitable threshold may be experimentally determined and/oroptimized to distinguish between properly functioning and improperlyfunctioning particulate filters. It will be appreciated that thethreshold voltages shown in Table I are merely exemplary, and that asensor may have a different optimum threshold voltage depending upon thesensor construction, the electronics used to amplify and/or process thesignal from the sensor, and other such factors.

FIG. 7 shows a schematic view of another embodiment of an engine,generally at 700. Engine 700 includes a single particulate sensor 702positioned downstream of a particulate filter 704. Because engine 700includes no particulate sensor positioned upstream of the particulatefilter 704, method 500 may not be suitable for use with this engine.Instead, a method may be used that is based upon the signal fromparticulate sensor 702 and a measure of soot accumulation on the exhaustwalls estimated from the length of time prior to the exhaust flowtransient that the engine operates in conditions favoring particulateproduction and at relatively constant exhaust velocity. A pass/failmetric for particulate filter performance may be given by the sum of theintegrated sensor output during a flow burst scaled by the sootaccumulation before the event, such sum carried out over a calibratablenumber of events.

FIG. 8 shows, generally at 800, a flow diagram of an exemplaryembodiment of a method of monitoring the performance of a particulatefilter with a single particulate sensor positioned downstream of theparticulate filter. Generally, method 800 includes first estimating orotherwise determining a measure of soot accumulation in the exhaustsystem upstream of the particulate filter, and then detecting amagnitude of a signal from the downstream particulate sensor upon theoccurrence of an increase in a rate of exhaust flow. A performance ofthe particulate filter may be determined by a comparison of the signalfrom the particulate sensor and the measure of soot accumulation. Itwill be appreciated that the specific steps and order of steps shown inFIG. 8 are merely exemplary, and that any other specific steps and/ororder of steps may be used.

Method 800 first includes, at 802, initializing variables and constantsused in the method, and then, at 804, sampling the various enginesensors and inputting the sampled values into controller 12. Next, anabsolute value of the rate of change of the exhaust flow is monitoredand compared, at 806, to a first threshold rate of change Th_dfmin. Anabsolute value of the rate of change of the exhaust flow below Th_dfminindicates steady state operation. If the absolute value of the rate ofchange of the exhaust flow is below Th_dfmin, method 800 next includes,at 808 verifying that an event flag that signals an exhaust flowincrease event flag is set at 0 (which signifies that no exhaust flowincrease event is occurring), and then increasing a soot accumulationcounter Soot_acc by a determined amount. The amount that the sootaccumulation counter is increased may be determined, for example, from apredetermined mapping of exhaust particulate concentrations as afunction of various engine variables. Alternatively, a measure orestimate of the soot accumulation in the exhaust system upstream of theparticulate filter may be determined in any other suitable manner.

Method 800 continues to loop through 804, 806 and 808, therebyincreasing the soot accumulation total Soot_acc until an increase inexhaust flow is detected. If the absolute value of the rate of change ofthe exhaust flow is greater than the threshold Th_dfmin, then it isdetermined, at 810, whether the increase is greater than a secondpredetermined rate of change threshold Th_df. If the rate of change isgreater than the threshold Th_df, this indicates that the rate of changeis great enough to cause a burst in exhaust particulate concentrationlarge enough for diagnostic purposes. In this case, method 800 nextincludes, at 812, changing the exhaust flow increase event flag to avalue of 1, and then initializing a timer Tdel to a predetermined valuedel_th. The function of the timer Tdel is described in below.

After initializing the timer Tdel at 812, method 800 next includes, at814, determining whether the reading from the particulate sensor locateddownstream of the particle filter is above a threshold level Sen_th,wherein the threshold level Sen_th represents a threshold, for example,that is sufficiently high to distinguish sensor noise from an increasein particulate matter concentration caused by the increase in the rateof exhaust flow. If the sensor reading is below the threshold levelSen_th, then method 800 loops back to 804. On the other hand, if thesensor reading is above the threshold level, then method 800 nextincludes, at 816, beginning to integrate the sensor reading as afunction of time. This is indicated by the equation Even=Even+sensor(i),wherein the term “Even” represents the integrated signal from the sensoras a function of time, and wherein “sensor(i)” represents the ithincremental sensor reading. Method 800 then continues to loop through804, 806, 810, 812, 814 and 816 while the rate of change of exhaust flowremains above the threshold Th_df.

At some point, the rate of change of exhaust flow drops below thethreshold Th_df. During this transitional period, the absolute value ofthe rate of change still may remain above the threshold Th_dfmin. Underthese conditions, method 800 proceeds through steps 804 and 806 to step810. At 810, however, if the rate of change of exhaust flow is less thanthe threshold Th_df, then the status of the event flag ev_flg isdetermined at 818, and the value of the time counter Tdel is decreasedby one if the event flag is equal to 1. Next, it is determined, at 820,if Tdel has been decreased to zero, and if Tdel is not yet zero, thenthe integration of the sensor reading continues at 816. In this manner,method 800 continues to perform the integration of the signal for apredetermined period equal to del_th after the rate of change of exhaustflow drops below the threshold rate of change Th_df.

Eventually, the time counter Tdel is reduced to a value of zero. Oncethis occurs, method 800 detects the zero value of Tdel at 820, and thenproceeds to 822. At 822, an event counter N_ev is increased by one,indicating that the current exhaust increase event is closed. Next, aleakage index is calculated by dividing the integrated sensor signal(Even) from the closed event by the determined measure of sootaccumulation (Soot_ac) existing prior to the beginning of the closedevent, and adding this quotient to the total leakage index(Leakage_index). Next, the soot accumulation variable Soot_ac, thesignal integration variable Even, and the event flat ev_flg are eachreset to zero, and the time counter Tdel is reset to the predeterminedinitial value del_th. Method 800 then loops, at 824, back to 804 tobegin anew. In this manner, method 800 totals the leakage index over apredetermined number N_ev of exhaust rate increase events. Once N_evreaches a predetermined maximum count, method 800 next includes, at 826,comparing the value of the variable Leakage_index with a predeterminedmaximum value L_index_max. If Leakage_index is less than L_index_max,this indicates that the particulate filter is performing adequately, asshown at 828. On the other hand, if Leakage_index is greater thanL_index_max, this indicates that the particulate filter is notperforming adequately, as indicated at 830. Upon determining that theparticulate filter is not performing adequately, controller 12 mayilluminate a MIL or other such indicator to prompt a user of theautomobile to have the particulate filter replaced.

FIG. 9 illustrates, generally at 900, a schematic representation (at902) of a signal from the downstream particulate sensor, a rate ofchange in exhaust flow (at 904), and a derivative of the rate of changeof exhaust flow (at 906) as a function of time. Furthermore, anexemplary value of the flow rate-of-change threshold Th_df is shown at908 and an exemplary value of the sensor threshold sen_th at 910. Theareas under the sensor threshold curve that are integrated during theperformance of method 800 are shown at 912 a, 912 b and 912 c. Firstreferring to the area 912 a, this integration is triggered by anincrease in the rate of flow that exceeds rate-of-change threshold 908,and by an increase in the sensor output to a level above sensorthreshold 910. The entire area under that portion of the curve thatexceeds sensor threshold 910 is integrated, as the time counter Tdelutilized in method 800 continues the integration for a sufficient timeperiod after the increase in the rate of exhaust flow drops below therate-of-change threshold 908.

Next referring to areas 912 b and 912 c, it can be seen that only partof the areas under the corresponding exhaust flow peaks that are abovethe sensor threshold 910 are integrated for each of these areas. This isbecause the rate of change of exhaust flow drops below rate-of-changethreshold 908 and time counter Tdel advances to zero before the sensoroutput drops below sensor threshold 910. Therefore, as shown in FIG. 9,method 800 integrates the output of the particulate sensor 702 only forthe period during and directly after a sudden increase in exhaust flow.Method 800 may continue integrating the particulate sensor output forany suitable interval after the rate of exhaust flow change drops belowrate-of-change threshold 908, or may stop integrating at the time therate of exhaust flow change drops below threshold 908.

The apparatuses and methods described herein may also be used in otherapplications than monitoring a performance of a particulate filter. Forexample, the disclosed apparatuses and methods may also be used toperform diagnostics on the operability of the particulate sensors. Oneexample of a method of performing a diagnostic determination on theoperability or performance of a particulate sensor or sensors is asfollows. First, a measure of a degree of particulate matter accumulationon a wall of the exhaust system upstream of the particulate sensor orsensors may be determined, for example, as described above in regard toprocesses 804-808 in FIG. 8. Next, an increase in exhaust flow may bedetected (directly or indirectly via other engine operating conditions),and the rate at which the exhaust flow increases may be compared to athreshold rate of exhaust flow increase, as described above in regard toprocess 810 in FIG. 8. If the rate of exhaust flow increase does notmeet a predetermined relationship to the threshold rate of exhaust flowincrease, then the diagnostic may be deferred until a subsequent exhaustflow increase that meets the predetermined relationship. Any suitablerelationship may be used as the predetermined relationship between thedetermined rate of exhaust flow increase and the threshold rate ofexhaust flow increase. Examples of suitable relationships include, butare not limited to, the determined rate of exhaust flow increase beinggreater than or equal to the threshold rate of exhaust flow increase.

On the other hand, if the rate of exhaust flow increase meets thepredetermined relationship to the threshold rate of exhaust flowincrease, then an output of the particulate sensor may be compared to apredetermined diagnostic output threshold. If the output of theparticulate sensor has a predetermined relationship to a predetermineddiagnostic sensor output threshold, then it may be determined that theparticulate sensor has degraded, and an alert may be activated to alerta vehicle operator of this condition. On the other hand, if the outputof the particulate sensor does not have the predetermined relationshipto the diagnostic sensor output threshold, then it may be determinedthat the particulate sensor has not degraded significantly. Any suitablerelationship may be used as the predetermined relationship between thesensor output and the predetermined diagnostic sensor output threshold.Examples include, but are not limited, to, sensor outputs having anabsolute magnitude or absolute change in magnitude equal to or below thepredetermined diagnostic sensor output threshold. The output of thesensor used in this comparison may be an instantaneous output, anintegration of the sensor output over a time interval, engine cycleinterval, or other interval, or may be any other suitable representationof the output of the subject sensor.

The threshold rate of exhaust flow increase may have any suitable value.Examples of suitable threshold rates of exhaust flow increase include,but are not limited to, threshold rates of sufficient magnitude toconsistently dislodge detectable amounts of particulate from the exhaustsystem walls. Furthermore, in some embodiments, different thresholdrates of exhaust flow increase may be used for different determinedmeasures of particulate matter accumulation on the exhaust system wallupstream of the subject sensor, while in other embodiments, a singlethreshold rate of exhaust flow increase may be used irregardless of thedetermined measure of particulate matter accumulation on the exhaustsystem wall. Where different threshold rates of exhaust flow increaseare used for different determined measures of particulate matteraccumulation on the exhaust system wall, a table or mapping of aplurality of determined measures of particulate matter accumulation onthe exhaust system wall and corresponding threshold rates of exhaustflow increase may be stored in memory on controller 12.

Likewise, the diagnostic sensor output threshold may have any suitablevalue. Examples of suitable values for the diagnostic sensor outputthreshold include, but are not limited to, values that may allow for adegree of insubstantial degradation in performance of the subject sensorwithout triggering an alert. Furthermore, in some embodiments, differentdiagnostic sensor output threshold values may be used for differentdetermined measures of particulate matter accumulation on the exhaustsystem wall upstream of the subject sensor, while in other embodiments,a single diagnostic sensor output threshold may be used irregardless ofthe determined measure of particulate matter accumulation on the exhaustsystem wall. Where different diagnostic sensor output thresholds areused for different determined measures of particulate matteraccumulation on the exhaust system wall, a table or mapping of aplurality of determined measures of particulate matter accumulation onthe exhaust system wall and corresponding diagnostic sensor outputthresholds may be stored in memory on controller 12.

A sensor diagnostic such as the diagnostic described above may beperformed for a sensor positioned either upstream or downstream of aparticulate filter. However, where the particulate filter is functioningproperly, insufficient particulate matter may reach a sensor positioneddownstream of the particulate filter to perform the diagnostic reliably.Therefore, controller 12 may be configured to perform a diagnostic of asensor positioned downstream of the particulate filter only underconditions during which quantities of particulate matter that aredetectable by the downstream particulate sensor are dislodged from theexhaust system walls, for example, after periods during whichsufficiently high measures of particulate accumulation on the exhaustsystem wall are likely to have occurred as determined from engineoperating conditions during the period, and/or during periods ofsufficiently high exhaust flow rate increase.

The embodiments of systems and methods disclosed herein for monitoring aperformance of a particulate filter are exemplary in nature, and thesespecific embodiments are not to be considered in a limiting sense,because numerous variations are possible. The subject matter of thepresent disclosure includes all novel and non-obvious combinations andsubcombinations of the various particulate sensor and exhaust systemconfigurations, systems and methods for monitoring the performance ofthe particulate filter via the various particulate sensors, and otherfeatures, functions, and/or properties disclosed herein. The followingclaims particularly point out certain combinations and subcombinationsregarded as novel and nonobvious. These claims may refer to “an” elementor “a first” element or the equivalent thereof. Such claims should beunderstood to include incorporation of one or more such elements,neither requiring nor excluding two or more such elements. Othercombinations and subcombinations of the various features, functions,elements, and/or properties disclosed herein may be claimed throughamendment of the present claims or through presentation of new claims inthis or a related application. Such claims, whether broader, narrower,equal, or different in scope to the original claims, also are regardedas included within the subject matter of the present disclosure.

We claim:
 1. A method for determining performance of a particulatefilter in an exhaust system of an engine, comprising: accumulatingparticulate filter leakage over a predetermined number of exhaustevents, the exhaust events comprising a rate of change of exhaust flowexceeding a threshold value, particulate filter leakage based on aparticulate sensor located downstream of said particulate filter; andassessing particulate filter leakage in response to the accumulatedparticulate filter leakage, and where, in response to a decrease in therate of change of exhaust flow to a rate lower than the threshold value,continuing to integrate an output of said particulate sensor for apredetermined interval after the decrease.
 2. The method of claim 1,wherein assessing particulate filter leakage includes integrating saidoutput of said particulate sensor.
 3. The method of claim 2, whereinintegrating said output of said particulate sensor includes integratingsaid output of said particulate sensor for a predetermined intervalafter said rate of change of said exhaust flow exceeds the thresholdvalue.
 4. The method of claim 2, further comprising determining ameasure of particulate matter accumulation in the exhaust systemupstream of the particulate filter before detecting an increase inexhaust flow.
 5. The method of claim 1, wherein the particulate sensoris a downstream image charge sensor.
 6. The method of claim 1, whereinsaid rate of change of exhaust flow is detected from an engine operatingparameter change which is indicative of an increase in exhaust flow. 7.A system, comprising: an internal combustion engine; an exhaust systemfor transporting engine exhaust from the engine; a particulate filterdisposed in the exhaust system; a particulate sensor disposed downstreamof the particulate filter in the exhaust system, wherein saidparticulate sensor is an image charge sensor; and a controllerconfigured to assess leakage of the particulate filter in the exhaustsystem in response to a predetermined number of engine operatingparameter changes indicative of an increase in exhaust flow in theexhaust system equal to or greater than a threshold rate of change, andin response to an integrated value of an output of said particulatesensor exceeding a threshold value, wherein said controller integratessaid output of said particulate sensor over a predetermined period, andwherein said controller further detects when said increase in exhaustflow shifts to a rate lower than said threshold value and continues tointegrate said output of said particulate sensor for a predeterminedinterval after said increase in exhaust flow shifts to said rate lowerthan said threshold value.
 8. The system of claim 7, wherein thecontroller is configured to determine a measure of particulate matteraccumulation in the exhaust system upstream of the particulate filterbefore detecting said increase in exhaust flow.
 9. The system of claim7, further comprising an upstream particulate matter sensor positionedupstream of the particulate filter.
 10. The system of claim 7, whereinsaid controller detects said increase in exhaust flow by detecting anengine operating parameter change which is indicative of said increasein exhaust flow.
 11. A method for determining particulate filterleakage, comprising: integrating output of a particulate sensor locateddownstream of a particulate filter in response to an increase in exhaustflow greater than a threshold rate of change; and continuing tointegrate the output of the particulate sensor for a predeterminedinterval after the increase in exhaust flow shifts to a detected ratelower than the threshold rate.